1. Field of the Invention
This invention relates to a supercapacitor cell, particularly a cell that is capable of delivering a charge at high current density.
2. The Prior Art
Current high power capacitors rely on the charging and discharging of an electrolyte double-layer to store and deliver electrical energy. Most of these double-layer capacitors use high surface area carbon-electrodes which can have gravimetric areas of over 10.sup.7 cm.sup.2 g.sup.-1. However, the double-layer capacitance of these electrodes in aqueous or organic electrolytes, is limited to only about 10-150 micro uF cm.sup.-2.
The concept of using surface or absorbed redox couples in capacitor applications is known. These "supercapacitor" systems rely on faradaic processes to store and deliver charge. For these systems to function, the species formed on charging must be insoluble in the electrolyte or absorbed into the electrode surface so that the current achieved during discharge has no diffusion limitations. It is also advantageous to have the discharged species to remain on the electrode to facilitate charging but this is not required for the supercapacitor to function. Examples of redox processes which function as supercapacitor electrodes are underpotentially deposited hydrogen on platinum, underpotentially deposited lead on gold and insoluble transition metal oxides/hydroxides (e.g., RuO.sub.2 and IrO.sub.2).
For a further description of the above prior art systems, see Transition From "Supercapacitor" to "Battery" Behavior in Electrode Chemical Energy Storage, B. E. Conway, J. Electrochem. Soc., 138, 1539 (1991), which is incorporated herein by reference.
The above Conway Article is directed to the redox of hydrous oxides in aqueous electrolytes. These supercapacitors are described as being able to produce a capacitance of up to 2000 micro .mu.F cm.sup.-2.
However a supercapacitor with a considerably greater capacitance than that found in the prior art is desirable and there is need and market for one that overcomes the above prior art shortcomings.
There has non been discovered a supercapacitor cell with a greatly increased capacitance over those found in the prior art, which cell operates with a high rate of discharge and with redox reversibility.